Global freshwater discharge into the oceans is a major component of global water cycle and a crucial connection between the land and the ocean1. It directly affects the ocean water balance as the main water resource besides precipitation2. In the context of global climate change, atmospheric circulation and ocean current determine the pattern of continental water cycle by controlling the amount of water vapor entering and water stored on the land3,4,5, thus affecting regional runoff into the ocean6 and global biogeochemical cycle7. While the vertical flux circulation of global water cycle is accelerating8,9, the horizontal flux is decreasing, especially the inflow to the ocean10. The quantitative identification of continental discharge into the ocean, on a global scale and at different temporal and spatial scales, is essential to assessing changes in the Earth system11 and also important for tackling climate change.
At present, the assessment of continental discharge into world’s oceans mainly includes: (1) Runoff observation of major rivers, but there are still obstacles such as data continuity and sharing, and incomplete monitoring coverage of all continental watersheds that contribute runoff to oceans12,13; (2) Runoff simulation of major rivers based on numerical modeling, but it failed to render high-precision simulation results14 globally; and (3) Estimate of river discharge from basin to continental scales, relying on remote sensing inversion techniques such as the Gravity Recovery and Climate Experiment (GRACE) satellites15,16, but the research are limited by observation period and the temporal and spatial scales17.
To this end, this paper firstly defines, in a refined manner, the boundary of endorheic basins and the range of contributing watersheds before assessing continental discharge into the ocean. Based on the data of hydrology, precipitation, evaporation, land use and water withdrawal, kilometer-scale global continental discharge from 1951 to 2020 is estimated through interpolation, extension and transplantation of continental discharge coefficient, while considering climatic-ecological-hydrological zoning. The temporal and spatial variances of terrestrial discharge into various oceans are also examined according to contributing watersheds.
Overview of terrestrial watersheds and water discharge into world’s major oceans
From 1951 to 2020, global continental discharge into the oceans registered an annual average of 39.33 trillion m³ with discharge per unit area reaching 0.34 trillion m³, which is equivalent to 7.7% of global precipitation or 36.9% of land precipitation (excluding Antarctica). The Atlantic (AT) Ocean received the largest amount of discharge, with an annual average of 20.09 trillion m³, accounting for about half (51%) of the global continental discharge. The discharge contains four of the world's six major rivers (Amazon, Congo, Orinoco, and Parana). The Arctic (AR) Ocean received the smallest discharge (4.2 trillion m³ per year), approximately taking up 11% of the global discharge. There are obvious spatial differences in continental discharge to the oceans, Asia and South America contributed to 60% of the global continental discharge. More specifically, discharge from South America and Africa into the Atlantic Ocean and discharge from Asia into the Pacific (PA) Ocean represented 28%, 10%, and 16% respectively, while discharge from other continents to the oceans accounted for 46% in total.
Global river discharge from continents to oceans is spatially uneven and concentrated in low-latitude areas. In terms of latitude zoning, on average, about 35% of river runoff flows into the equatorial regions between 10°N and 10°S. With the exception of the Yenisei, Leine, Obo, and Amur River, most of the world's major rivers are located in the lower latitudes with the highest precipitation rates18.
The multi-year average runoff modulus for global continental discharge to the ocean is 0.34. The maximum is 0.87 entering the Atlantic Ocean, that is, 20% of the global land area provides 51% of global terrestrial discharge. The minimum is 0.1 flowing into the Indian (IN) Ocean, with 44% of the global land area contributing only 13% of the global discharge. By continent, South America presents the largest average discharge modulus of 0.64, noticeably showing a 0.65 in the discharge to the Atlantic Ocean. Oceania sets the smallest average of 0.22, with a low level of 0.06 to the Indian Ocean and a high level 0.6 to the Pacific Ocean, just after the discharge from South America into the Atlantic Ocean, providing 13% of the water with 2% of the land area.
Temporal variation characteristics of global terrestrial water discharge into the ocean
Global continental discharge into the oceans has changed sharply over the past seven decades, rising before 2000 and declining thereafter, then falling to record low at a rapid rate in the last decade. The world annual average is 0.93–2.78 trillion m³ smaller than the 1951–2000 average, making 2020 the lowest year since the mid-to-late 19th century. The average rate of decline stood at 0.2 trillion m³/10a since 1950, but multiplied 26 times from 2011 onwards. During 1951-2000, the global discharge grew by 0.6 trillion m³/10a, but plummeted during 2001–2020, noticeably at a rate of 5.3 trillion m³/10a in the recent decade, resulting in post-2000 consecutive decrease in the discharge entering the sea. This also explains the inverted V-shaped change of data sequence.
Continental discharge into world’s major oceans shows a downward trend, with the exception of a growth in the Arctic Ocean. Over the past seven decades, continental discharge to the Atlantic, Indian, and Pacific oceans slumped at a rate of 0.45 to 0.58 times of the global decline rate, while discharge into the Arctic Ocean increased at a rate of 0.61 times. The largest decline rate occurred in the discharge from Asia to the Pacific Ocean, with 0.71 times that of the world. The smallest decline rate was observed in the discharge from Australia to the Pacific Ocean, which is only 0.03 times that of the world average. Continental discharge to the Atlantic Ocean hit record low for six consecutive years from 2016 to 2021, reaching the lowest level since 1951 that is about 18% below the 1999 average (the highest year).
There have been abrupt changing points in about 71% of the terrestrial watersheds around the world, among which 33% regions experienced abrupt change after 2000, which is consistent with the years when occurring steep drop of the global continental discharge. Continental discharge into world’s major oceans, except for the Arctic Ocean, has all experienced significant changes. The abrupt changes in the continental discharge to the Atlantic Ocean have a decisive effect on the mutations in the global continental discharge, both of which took place in 2000, slightly later than that in the Indian Ocean (1999) and earlier than that in the Pacific Ocean (2010).
A decrease trend of continental discharge emerged within about 97% of the mutant areas, noticeably in 43.83% of the areas. From 1951 to 2000 (before the abrupt changing point), global average annual continental discharge showed an evident upward trend within the 5% confidence interval, however, tended to decline significantly at a fourfold rate after the abrupt changing point. Due to such abrupt change, the multi-year average of global continental discharge dropped sharply from 39.89 trillion m³ to 37.94 trillion m³, with the rate of change nearly quadrupled, reaching nearly 1.95 trillion m³. Continental discharge into the Pacific, Indian, and Atlantic Ocean all decreased by more than 7%, with a pronounced decline of 10% for the Indian Ocean. The coefficient of variation (Cv) increased from 0.04 to 0.05, indicating an increase on inter-annual variation.
Attribution analysis of changes in global terrestrial water discharge into the ocean
Changes in precipitation, evaporation, and water use directly affect the amount of runoff into the ocean. Global continental discharge into the ocean is negatively correlated with precipitation and evaporation, and especially with artificial water withdrawal. About 99% of the areas rendered discharge in positive correlation with precipitation before mutations, and a larger proportion saw significant correlation after mutations. Due to such changes, areas with positive correlation between surface discharge and evaporation was reduced from 70% to 60%, while areas with negative correlation between surface discharge and artificial water withdrawal rose from 54% to 69%.
As a result of these abrupt changes, average annual precipitation in watersheds contributing runoff into the ocean increased by 7%, and the variance altered abruptly from 500 million m³/10a to negative 0.8 trillion m³/10a. As to evaporation, multi-year average rose by 11%, with transition from a negative rate of 0.04 trillion m³/10a to a positive rate of 3.25 trillion m³/10a. At the same time, multi-year average human water withdrawal intensified by 52%, with the rate slowing down 0.78 times from 0.37 trillion m³/10a. Among them, terrestrial discharge into the Pacific Ocean reduced by 9% as water withdrawal, precipitation, and evaporation in the contributing watersheds grew by 88%, 8%, and 12% respectively. That is to say, such reduction was driven by these three factors jointly. There has been a marked increase of water withdrawal in about 89% of the contributing watersheds around the world, mainly centralizing at densely populated and economically developed areas. This implies that these areas need more promising policies, stricter standards, and more effective measures to achieve global climate goals.
Increased continental discharge into the Arctic Ocean is directly related to global warming19,20, in addition to the extensive growth of wetland vegetation, the deposition of lowland peat sediment, and the influence of large-scale topographic factor, especially permafrost21. However, through natural variability, river runoff swelled mainly under the condition of atmospheric anticyclone22.
From 1951 to 2020, global continental discharge into the oceans changed sharply, witnessing an increasing before 2000 and a downward trend thereafter. It dropped at a faster rate in the recent decade, reaching the lowest level in history. Over the seven decades, changes in precipitation, underlying surface, and human water withdrawal seem to have caused anomalies in the runoff into the sea, but other factors such as overall seawater temperature rise and sea level changes23,24 may also lead to estuary changes, making continental discharge into the oceans different. To further complicate matters, great uncertainties still remain in how the ocean-land cycle as a whole responds to changes of the continental discharge into the ocean. On a global scale, this study focuses on research at annual and inter-annual scales. In-depth research at monthly and sub-monthly scales is still needed, so the next step is expected to aim at monthly-scale research in a refined manner.